In the field of radio communication systems, it is a well-known problem that the power amplifiers present in transmission equipment operate in a non-linear fashion when the power amplifiers are operated near their peak output. As a result, the power amplifier introduces significant signal distortion that can appear in various forms. For example, if more than one signal is input into the power amplifier or power amplification stage, its non-linear characteristics can cause an undesirable multiplicative interaction of the signals being amplified, and the power amplifier""s output can contain intermodulation products. These intermodulation products cause interference and crosstalk over the power amplifier""s operational frequency range.
In power amplifier design, there is a trade off between distortion performance and efficiency. Linear amplifiers that operate under xe2x80x9cClass Axe2x80x9d conditions create little distortion but are inefficient, whereas nonlinear amplifiers operated under xe2x80x9cClass Cxe2x80x9d conditions are reasonably efficient but introduce significant distortions. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, nonlinear amplifiers are largely preferred, leaving the user with the problem of distortion.
In order to employ nonlinear power amplifiers, techniques have been used to improve linearity and thereby reduce the effects of interference and crosstalk. Linearity can be achieved by application of various linearization techniques that reduce the distortion caused by nonlinear amplification. Conventional amplifier linearization techniques can be broadly categorized as feedback, feedforward, or predistortion.
The last mentioned technique, predistortion, intentionally distorts the signal before the power amplifier so that the non-linearity of the power amplifier can be compensated. According to this technique, linearization is achieved by distorting an input signal according to a predistortion function in a manner that is inverse to the amplifier characteristic function. The predistortion technique can be applied at radio frequency (RF), intermediate frequency (IF), or at baseband.
In the baseband domain, the input signal information is at a much lower frequency, allowing digital methods to be employed. The predistortion function is applied to the input signal with the resulting predistorted signal being upconverted to IF and then finally to the RF carrier frequency. It is also possible to apply adaptive predistortion techniques where feedback from the output of the amplifier is used to update and correct the predistortion function.
The form of the predistortion function is dependent upon the model used to characterize the output of the amplifier. Predistortion functions in the baseband domain are typically implemented as a table of gain and phase weighting values within a digital signal processor. A Cartesian feedback method employs a quadrature representation of the signal being amplified. The incoming quadrature signals I and Q are compared to the feedback quadrature signals. Thus, there are two sets of coefficients, one for each quadrature channel, that are being updated to model the predistortion characteristics. In this manner, gain and phase non-linearities within the amplifier can be compensated. Performance is dependent on the size of the look up table and the number of bits used to represent the signal. Better performance and more adaptivity is achieved with larger look up tables and more bits albeit at the expense of longer processing times.
Predistortion functions are also modeled as polynomials. Here, the complex polynomial must be able to characterize the inverse of the amplifier, which may not analytically exist and which must then be approximated. In order to accurately estimate the inverse, the polynomial requires high order terms, with associated quantization errors and a less accurate polynomial fit.
Accordingly, there is a need for a power amplifier predistortion system that can quickly and efficiently obtain an optimum predistortion function for non-linear amplifiers.
Disclosed is an apparatus and method for calculating the predistortion function from a power amplifier. A predistortion module generates a predistorted signal in response to an input signal and predistortion coefficients. The predistortion function calculating module generates the predistortion coefficients in response to given amplifier characteristics. Polynomial fitting is used to obtain the predistortion coefficients. For example, rather than analytically calculating the inverse to an amplifier characteristic curve, a number of points along the amplifier characteristic curve are used to determine a predistortion function having a corresponding order. In certain embodiments, the x- and y-axis terms of the amplifier characteristics curve are switched to produce points along a predistortion curve. The predistortion function calculating module calculates a polynomial function that connects the points. Thus, the predistortion function can be more easily calculated.
In exemplary embodiments of the present invention, the predistorted signal is represented by single or multiple sectors having multiple points and then predistortion coefficients are calculated for each sector. Importantly, employing multiple sectors can improve modeling effectiveness by decreasing the number of higher order terms needed to find the optimum coefficients for the predistortion function. Execution rates are increased by eliminating higher order terms, thereby reducing the need for more extensive hardware. Thus, the present invention provides a flexible method of implementation that provides improvements in speed, accuracy, and cost.
By improving the ability to model the power amplifier predistortion function, power amplifiers can be operated in the nonlinear region near saturation, yet suppress undesirable intermodulation products. Resort to a larger amplifier, to keep operation within the linear region, is avoided. Power amplifier sizes are kept small with associated cost savings, particularly important in the field of wireless communications.
The above factors make the present invention essential for effective power amplifier predistortion.